Northern fur seal description

The northern fur seal (Callorhinus ursinus) belongs to the family known as ‘eared seals’ (Otariidae), which includes the fur seals and sea lions. This group differs from the true seals in having small external earflaps and hind flippers that can be turned to face forwards. Together with strong front flippers, this gives them extra mobility on land and an adult fur seal can move extremely fast across the beach if it has to. They also use their front flippers for swimming, whereas true seals use their hind flippers (3).

Fur seals show a considerable size difference between the sexes. A bull northern fur seal is a big, heavy animal, over 2 metres in length and weighing as much as 270 kilograms. Its fur varies in colour, ranging from reddish to black, and it has a thick neck and a mane (2). The females (cows) are much smaller, at less than 1.5 metres in length and weighing a fifth as much as the bulls (2). The colouring of the female differs too, with a silvery-grey back, reddish-brown on the front and a whitish-grey patch on the chest (4). Young northern fur seal pups are black with paler markings around the nose and mouth (2).

Related species

Northern fur seal biology

At the start of the breeding season, male northern fur seals begin arriving at their traditional breeding sites about a month ahead of the females, and start competing for breeding territories(4). These fights can be extremely violent, with each mature bull aiming to slash an opponent’s neck with his sharp canine teeth. Only the largest and heaviest bulls can hope to claim the title of ‘beachmaster’; the smaller younger males, who have no chance of competing with the fully-grown animals, occupy the fringes of the breeding territories(3).

The female northern fur seals arrive at the breeding grounds in mid-June and give birth to the pups, conceived the year before, some two days after their arrival. Within a week of the birth, the females will mate again (4). Males compete to secure as many females as they can within a harem, although it is thought that females are influenced by the presence of other females and the characteristics of the territory rather than the mere size and power of the male (2).

The beachmasters will continue to squabble and fight over females right through the breeding season, usually because these colonies are crowded, and wandering females sometimes stray into another male’s territory(3). Occasionally, younger males will attempt to steal a mating with a female and, if spotted by one of the beachmasters, they will be chased off (2). In order to ensure that their females are not claimed by another male, northern fur seal bulls do not feed throughout the breeding period and may eventually loose 20 percent of their body weight (4).

Female northern fur seals suckle their pups for up to ten days before returning to feed at sea, usually during the night. The female will stay at sea feeding for four to ten days, returning to feed the pup for one or two days. The female will do this for four months before leaving her youngster and migrating south, usually in late October (4). Fur seals feed on a variety of prey, including squid and pollock (4), and have also been recorded taking seabirds (2).

The fertilised egg within the female fur seal undergoes a four-month period of delayed implantation. This ensures that that the developing pup will be born at the right time the following year when the animals return to their breeding grounds. The pups will spend as long as 22 months at sea before returning to the beach where they were born (4). Fur seals mature between the ages of three and six, but males will probably not begin to breed for an additional three years (2).

Northern fur seal range

The northern fur seal ranges over most of the northern Pacific Ocean, as far south as southern California in the east and to central Japan in the west. Northwards, its range extends to the Bering Sea, north of the Aleutian Islands, and the Sea of Okhotsk, west of the Kamchatka Peninsula (4).

The main breeding grounds of the northern fur seal are on the Pribilof Islands of St. George and St. Paul in the southern Bering Sea. It also breeds on a number of other islands scattered throughout its range, principally the central Kuril Islands and Tyuleniy Island in the Sea of Okhotsk, the Commander Islands off the coast of Alaska, Bogoslof Island in the Aleutians and San Miguel Island off the coast of California (4).

Species with a similar range

Northern fur seal habitat

Although the northern fur seal spends most of the year at sea in the cold northern waters of the Pacific Ocean (2), its breeding grounds are on rocky coastlines close to the edge of the continental slope (4).

Northern fur seal status

Northern fur seal threats

The fur that protects the northern fur seal from the cold has led to this species being hunted for centuries. Although early native peoples made little impact on its numbers, the ‘discovery’ of the species in the 18th century was followed by commercial hunting that nearly led to the northern fur seal’s extinction by the end of the 19th century. In 1911 a treaty, the North Pacific Fur Seal Convention, was signed by the USA, Japan, Russia and the UK (acting for the Dominion of Canada), limiting hunting to immature males on land and banning all sea hunts (4).

Although serious commercial hunting of the northern fur seal has ended, the animals no longer enjoy the protection of the international treaty which lapsed in 1984 (5). Neither is the northern fur seal currently protected under the Convention on International Trade in Endangered Species (CITES) (6).

There is still some hunting permitted under licence in Canadian waters by native peoples only (5), and a similar agreement exists for natives of the Aleutian Islands (4). However, the main threats to the northern fur seal globally are now believed to be caused by entanglement in the nets of the Japanese squid fishing fleets and in the Bering Sea. Seals are also threatened by marine pollution such as plastic twine and waste packaging, as well as discarded trawl nets. The animals are very vulnerable to oil pollution and, with an increase in oil and gas exploration around several of their breeding grounds, there are fears that accidental oil spills and the inevitable industrial disturbance will affect northern fur seal populations (4).

Northern fur seal conservation

The North Pacific Fur Seal Convention lapsed in 1984 after an extension was vetoed by the USA (5), but all commercial hunting has ceased at sea and only limited numbers of northern fur seals are now taken under licence. It is believed the world population currently stands at about 1,350,000 animals, but, with no current international agreement, some experts fear that commercial hunting could start again at some time in the future. There is also evidence that the El Niño event of 1997 to 1998 affected the pup survival rate of the San Miguel Island breeding colony. A drop in the availability of fish led to 87 percent of young northern fur seals dying before they were weaned (4).

Authentication

Glossary

Delayed implantation

The process of a fertilised egg remaining unattached in the uterus for a period of time, therefore delaying the start of development.

El Niño

A natural phenomenon that happens every 4 to 12 years, and lasts for several months, when upwelling of cold, nutrient-rich water does not occur. This causes the warming of ocean surface water off the western coast of South America and causes die-offs of plankton and fish. It also affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world.

Fertilisation

The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.

Territory

An area occupied and defended by an animal, a pair of animals or a colony.

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